1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more specifically to the so-called air intake side secondary air supply system for an internal combustion engine having an atomatic choke valve and a fast idle system.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air-fuel ratio of the engine is controlled around a stoichiometric value (14.7:1 for example) by a feedback control in accordance with the composition of the exhaust gas and operating conditions of the engine since an optimum operation of the three-way catalytic converter is enabled at the stoichiometric air-fuel ratio. The air intake side secondary air supply system is a system for the feedback control of air-fuel ratio, in which the air-fuel ratio is controlled by varying the amount of a secondary air through a secondary air passage connected to the carburetor on the downstream side of the throttle valve. In the air intake side secondary air supply system, the feedback control of the air-fuel ratio is stopped by suspending the supply of the air intake side secondary air during a period of the cold operation of the engine, and in such a period when the throttle valve is substantially fully closed because the engine operation tends to be unstable during such periods. The cold operation of the engine is detected as such a state that the intake air temperature is below a first predetermined temperature level (18 deg. C. for example) and that the engine coolant temperature is below a second predetermined level (70 deg. C. for example) even though the intake air temperature is above the first predetermined level. By stopping the supply of the intake side secondary air, the air-fuel ratio is controlled to the rich side and a stable operation of the engine is secured. In the conventional arrangement, the condition for starting the feedback control of the air-fuel ratio after the cold start of the engine is detected when the throttle valve is opened wider than a predetermine level. However if the engine is equipped with an automatic choke valve and a fast idle system in the air intake system, the choke valve may remain closed when the throttle valve is opened by the fast idle system and the engine coolant temperature has reached a temperature level for the start of the feedback control of the air-fuel ratio. If the air intake side secondary air is supplied in such a state, the engine speed will be raised abruptly to cause a hunting, and the driveability of the engine will be greately spoiled.